The present invention generally relates to superconducting quantum interference magnetometers that utilize digital superconducting quantum interference devices for the measurement of feeble magnetic fields, and more particularly to a superconducting quantum interference magnetometer equipped with a plurality of measuring channels.
Superconducting quantum interference magnetometers that utilize the superconducting interference devices abbreviated hereinafter as SQUIDs are used as the essential device for measuring the extremely feeble magnetic fields produced by the biologic bodies and organs such as brain or heart. Particularly, there is a keen demand for a magnetometer equipped with a plurality of measuring channels for measuring the distribution of magnetic field in a short time.
In such multi-channel SQUID magnetometers, a number of digital SQUID sensors are arranged parallel with each other, wherein each SQUID sensor produces a series of output voltage pulses in response to the unknown magnetic flux that interlinks with a superconducting detection loop of the SQUID sensor. In combination with each SQUID sensor, there is provided a corresponding feedback circuit that produces a counteracting feedback magnetic flux in the detection loop such that the unknown magnetic flux is counteracted by the feedback magnetic flux. The magnitude of this feedback magnetic flux is increased stepwise in response to each output voltage pulse of the SQUID sensor until the unknown magnetic flux is totally canceled out. Upon the cancellation of the magnetic flux, the induction current induced in the superconducting detection loop disappears and the SQUID sensor stops producing the output voltage pulses. The measurement of the magnetic flux is achieved by counting the number of output voltage pulses thus produced by the SQUID sensor. On the other hand, the direction of the magnetic flux is determined by detecting the polarity of the voltage pulse. Such a SQUID magnetometer using the digital SQUID sensor provides various preferable features such as increased S/N ratio, easiness in processing the output data by digital processing systems, and the like. The inventor of the present invention has previously proposed such a digital SQUID magnetometer wherein the SQUID sensor and the feedback circuit are assembled into a single chip in the U.S. Pat. No. 4,947,118, which is incorporated herein as reference. Such a so-called single chip SQUID magnetometer incorporates both the SQUID sensor and the feedback circuit in the liquid helium bath and thus eliminates the feedback conductor extending between the SQUID sensor in the liquid helium bath and the feedback circuit provided conventionally in the room temperature environment. Thereby, the problem of penetration of heat from the room temperature environment to the liquid helium bath through the feedback conductor is eliminated and the consumption or evaporation of the liquid helium used for maintaining the SQUID device at the superconducting state, is significantly reduced.
In constructing a multi-channel magnetometer using such a digital SQUID sensor, there is an obvious approach shown in FIG. 1, wherein a number of single chip SQUID magnetometers, each comprising a SQUID sensor such as the sensor 1a-1n, a corresponding feedback circuit such as the circuit 2a-2n, and a feedback path such as the path 3a-3n, are provided parallel with each other and connected to a processing and display unit 5 for digital processing of the output pulses and display of the result of measurement. Thus, the processing and display unit 5 receives the output voltage pulses of the SQUID magnetometers through a parallel output conductors 4a-4n when there are n such magnetometer channels.
In the construction of FIG. 1, it should be noted that the parallel conductors 4a-4n extend from the processing unit 5 operated at the room temperature to the SQUID magnetometers operated at the liquid helium temperature. In other words, the conductors 4a-4n extend across a wall of a liquid helium container in which the SQUID magnetometers are contained. Thereby, there arises a problem of heat penetrating into the liquid helium through these conductors. It should be noted that the number of conductors 4a-4n corresponds to the number of the channels. With increasing number of channels, this effect of penetration of heat and the associated problem of excessive consumption of liquid helium becomes a serious problem in the actual use of the SQUID magnetometer.
On the other hand, there is another known construction of multi-channel SQUID magnetometer as shown in FIG. 2, wherein the output of the SQUID sensors 1a-1n are sent to a multiplexer 11 provided in the room temperature system for a time-divisional multiplexing. The output of the multiplexer 11 is supplied to a feedback circuit 12 also operated in the room temperature system, and the feedback signal produced by the feedback circuit 12 is once stored in a memory 13 for each channel under control of a controller 15. Further, the feedback signal is fed back to the SQUID sensors 1a-1n from the memory 13 via feedback conductors 14a-14n. The memory 13 further supplies the feedback signal representing the detected polarity and magnitude of the magnetic flux to a processing unit 17 for processing and displaying the result of measurement.
In this apparatus, too, the problem of penetration of the heat is not eliminated. Particularly, as there are n additional conductor strips connecting the SQUID sensors and the multiplexer 11, the problem of evaporation of liquid helium is deteriorated rather than improved. Even when one could design the multiplexer 11 by using a Josephson device and thus succeeded in providing the multiplexer 11 in the liquid helium bath together with the SQUID sensors 1a-1n, the problem of penetration of heat through the feedback conductors 14a-14n remains unsolved.